Communications systems typically include a variety of devices (e.g., filters, mixers, amplifiers, integrated circuits, and so forth). Communications systems are useful for transmitting information (e.g., voice, video, data) relayed by means of wireless links, twisted pair, optical fibers, and so forth. As wireless communications systems become more advanced, signals are being transmitted at higher frequencies (e.g., PCS, ISM, etc). As systems are continually developed in response to market pressures, the demand for increased performance and reduced size intensifies. Market forces demand increased integration and reduction of component size.
Resonators such as Bulk Acoustic Wave (BAW) resonators are important components in the fabrication of bandpass filters and other related semiconductor devices. The BAW resonator is a piezoelectric resonator that essentially comprises a film of piezoelectric material (e.g., a crystalline AlN film), deposited between at least two electrodes. Upon application of voltage to such a structure, the piezoelectric material will vibrate in an allowed vibrational mode at a certain frequency. Piezoelectric resonators are thus useful in discriminating between signals based on frequency diversity (e.g., a bandpass filter), and in providing stable frequency signals (e.g. as in a frequency stabilizing feedback element in an oscillator circuit).
Typically, the performance of the resonant frequency of the piezoelectric resonator will depend upon the composition, thickness, and orientation of the piezoelectric material. The resonant frequency of a piezoelectric material is typically inversely proportional to its thickness; thus, for piezoelectric resonators to operate at high frequencies {e.g., frequencies greater than .about.700 Megahertz (MHz)}, the thickness of the piezoelectric film must be reduced to a thin film (e.g., having a thickness ranging from about 500 nm to about 10 .mu.m). The performance of a piezoelectric resonator is dependent on the crystalline orientation of the atoms comprising the piezoelectric film. The induced strain (i.e., stress wave) in a piezoelectric film in response to applied voltage (i.e., electric field) can only occur from the advantageous alignment of the atomic dipoles within the piezoelectric film. An example of an advantageous film orientation is &lt;002&gt; of AlN perpendicular to the substrate. A pulse DC sputtering method for depositing thin films of piezoelectric materials such as aluminum nitride (AlN) is described in U.S. patent application Ser. No. 09/145,323 to Miller et al., "Pulse DC Reactive Sputtering Method for Fabricating Piezoelectric Resonators," filed Sep. 1, 1998, assigned to the present assignee and incorporated herein by reference. In Miller et al., the quality of the piezoelectric films is improved with the techniques used to deposit the films themselves.
As may be appreciated, those in the field of communications systems and components continue to search for new methods for increasing system performance and integration. In particular, it would be advantageous to provide new methods for improving the quality of piezoelectric films. These and further advantages of this invention may appear more fully upon considering the detailed description given below.